Start with a song containing a few tracks, one of which has consistent timing, such as a drum loop. Here, the peaks in the waveforms don’t line up.
Choose the track you want to align the others to and hold the cursor over the left-edge of its track header. Click the star to set that track as the Groove Track.
A dialog box will appear asking if you want GarageBand to go ahead and analyse the audio within the project. Click Continue.
Another window may appear, asking if you want to “Enable Follow Tempo & Pitch?” Click continue to enable timing adjustments to be made.
The Groove Track’s star turns yellow. The waveforms of the edited tracks have shifted so that everything looks in line with the peaks in the Groove Track.
Any tracks that don’t require correction can be restored to their original timing by deselecting the checkbox to the left of their name.
If the results are still a bit patchy, you can use Flex Time to quickly correct anomalies. Double-click the problem region to open it in the Audio Editor pane.
Hover the cursor in the upper-half of the waveform and click the note or beat you want to adjust. Drag the vertical flex marker to shift the beat around.
The Groove Matching process is non-destructive, so the project’s original timing can always be restored. Just click the yellow star again to disable the feature.
Click Image to Enlarge: